docs: def from_df(cls, data, entities=None, source='contrast'):
          ''' Create a Collection from a pandas DataFrame.
  
          Args:
              df (DataFrame): The DataFrame to convert to a Collection. Each
                  column will be converted to a SimpleVariable.
              entities (DataFrame): An optional second DataFrame containing
                  entity information.
              source (str): The value to set as the source for all Variables.
  
          Returns:
              A BIDSVariableCollection.
          '''
          variables = []
          for col in data.columns:
              _data = pd.DataFrame(data[col].values, columns=['amplitude'])
              if entities is not None:
                  _data = pd.concat([_data, entities], axis=1, sort=True)
              variables.append(SimpleVariable(name=col, data=_data, source=source))
          return BIDSVariableCollection(variables)